The present invention relates generally to the monitoring of leakage currents in a transmission system and, more particularly, to a sensor for accurately detecting and monitoring leakage current in suspension type insulators.
In many countries, including the US, wood is utilized as part of the line insulation to improve the basic insulation level (BIL) of the line, as it has been recognized that the lighting performance of transmission lines can be improved by utilizing the wood support or pole. This has, however, not been without problems as there were, and still are, many instances of fires of the wood poles and cross arms caused by low frequency leakage current and sparking on the wood from sources such as leakage current due to insulator contamination despite mitigation measures being taken.
Insulators installed on transmission and distribution systems are exposed to contamination, for example from marine salt, road salt, and industrial pollutants. This contamination can result in flashover of the insulator, usually under light wetting conditions, e.g. condensation, when the salts and water mix to become a conductive electrolyte. Flashover is a problem in that it results in an outage which interrupts power to a utility company's customers.
When the salts on the surfaces of an insulator become wet they form an electrolyte which is conductive. Since the one end of the insulator is energized, and the other end is grounded, currents flow along the insulator surfaces. If these currents are large enough, arcing will occur (called dry band arcing). This arcing will either extinguish or grow to result in a flashover. The magnitude of leakage currents that occur under dry band arcing conditions provide an indication of the risk of the insulator flashing over. If insulators are at risk of flashover due to contamination build-up, utilities can wash the insulators or take other measures, such as redirect power to other transmission and distribution assets.
In addition when composite (also called polymer or non-ceramic) or coated insulators are used the leakage currents and arcing on the surface can degrade the rubber material. By monitoring the leakage currents an indication can be obtained as to the level of degradation.
Some commonly used mitigation measures are listed below; however, none of these mitigation measures provide a means for monitoring and pinpointing potential leakage current problems so that a utility company can take preventative measures.
1. Wrapping metal bands around the wood pole and connecting it to the insulator hardware. This method has the advantage that the reduction in the “insulated” wood path lengths (used as improvement for the BIL) is limited. The conductor material used for this purpose should be compatible with other hardware not to cause corrosion.
2. A small guard electrode, in the form of a coach screw or a multi-spiked plate (e.g. gang-nail), is fastened to the wood outside the rain shadow area and bonded to the insulator hardware. This method has a minimal effect on the BIL of the structure.
3. Application of conducting paint to cover the high resistance zones around metal to wood interfaces. This method has a minimal effect on the BIL of the structure.
4. Bonding of the insulator hardware together with a conductor. The intension with this bond wire is to “balance” the leakage current so that only a small residual current will flow in the pole. There are two variants to this scheme:                (a) The insulator bases are connected together but not grounded. The ground lead terminates some distance away to realize the required BIL phase-to-ground for induced lightning surges.        (b) The insulator bases can be bonded together and connected to ground. In this case the wood is not utilized anymore as part of the line insulation against lightning. Also here it is important that the material of the bond wire is selected to be compatible with the other hardware used to prevent corrosion.        
5. An extension of the previous method is to use steel cross-arms to bond the insulator bases together. The steel cross-arm can either be grounded or be left floating depending on whether or not the utility wants wood as part of the line insulation for lightning performance.
6. Finally the insulators used can be upgraded to those with an improved contamination performance. For example porcelain insulators can be replaced by hydrophobic silicone rubber composite insulators. Other options include regular insulator cleaning or the application of silicone grease to insulators.